It is known to use the watermarking technique to make security paper, i.e. documents that must not be falsified, in particular such as bank notes, payment means, identity documents, travel tickets, and tickets for cultural or sporting events.
The purpose of having a watermark present is to make it impossible to reproduce the document by optical means such as photocopying, photography, or scanning, because the medium on which the copy is reproduced does not include the watermark of the original document.
Watermarked papers are also used for decorative purposes, particularly for prestige writing and printing papers.
Watermarks are conventionally obtained by an operation of molding or embossing the wet sheet derived from the aqueous suspension of cellulose fibers during manufacture of the paper. At this stage of manufacture, the cellulose fibers migrate easily through the aqueous suspension so that the effect of the above operation is to concentrate fibers in thicker zones of the sheet and to disperse them in thinner zones, such that after the sheet has dried, and when it is observed in transmitted light, the sheet has pale zones that are poor in fibers and dark zones that have a high density of fibers.
The pale zones are referred to as zones of low optical density, lower than that of the “parchment”, i.e., in the meaning of the present application, the non-watermarked portion of the paper, and the dark zones are referred to as zones of high optical density, higher than that of the paper.
Depending on the local thickness of the watermarked zone, a desired density of fibers is obtained that corresponds to the looked-for tone shade. The pattern that appears thus includes an entire scale of tone shades, which is why such conventional “embossed” watermarks can be referred to as “shaded” watermarks.
Conventional shaded watermarks require expensive means because for each watermark pattern it is necessary to use a specific watermarking roll or wire, and these are in any event difficult to make.
Furthermore, the fact that conventional watermarked papers include regions of greater thickness gives rise to various problems.
Firstly, it slows down manufacturing rates because it is necessary regularly to stop machines that are designed for handling flat paper, in particular machines for transforming and for printing. Guillotining in particular is made difficult because the extra thicknesses of the watermarks give rise to deformation of the paper.
Another difficulty appears on printing since the way in which ink prints on the thicker zones is difficult to control, such that it is generally preferred to avoid printing on them and to print only outside the watermarked zones.
To compensate for these regions of extra thickness, the person skilled in the art puts spacers between the sheets. Such compensation is necessary to enable the sheets to be processed properly, but it requires additional and fiddly work to be performed which slows down processing of the sheets, particularly during printing.
In order to avoid those problems, attempts have already been made to produce pseudo-watermarks by using techniques that are less expensive, that conserve the initial thickness of the paper, and that provide a visual appearance that is close to that of a conventional watermark.
One known technique for making pseudo-watermarks consists in causing a composition to penetrate in or be printed on determined zones of the paper to modify the transparency of the paper and thus provide pale zones and dark zones like a watermark.
The drawback associated with that technique is that the compositions used spoil the surface properties of the paper, in particular its suitability for receiving printing inks, and the result obtained does not make it possible to obtain the same fineness and variation in brightness as in a conventional watermark.
Furthermore, such pseudo-watermarks are very easy to counterfeit since impregnating a sheet of paper with an appropriate composition is within the competence of numerous counterfeiters.